There is no method to extend the date of delivery.
However you can open "product not received" claim on the last day. When you open the claim ebay would give 5 days. If you believe that you would receive the product in 5 days you can close the claim when you receive the product.
Since you and seller are in good talking terms you can probably not open the case and believe that the product would be delivered. Or you can open a case and if the product is not delivered ebay would refund the payment. You can assure the seller that you would send the amount once you receive the product.